A ureter is a tubular passageway in the body that conveys urine from a kidney to a bladder. Blockages of the ureter may cause hydronephrosis, a condition characterized by damming of urine in the kidneys, resulting in swelling and inflammation of the upper urinary tract and the kidneys. This may lead to kidney dysfunction and eventual necrosis. Ureteral blockages, also called ureteral strictures, may be congenital or acquired. Acquired blockages may be caused by disease, injury, kidney stones, and tumors. Both congenital and acquired blockages generally require medical intervention, for example, the use of ureteral stents.
Ureteral stents are used to facilitate urinary drainage from the kidney to the bladder in patients having a ureteral obstruction or injury, or to protect the integrity of the ureter in a variety of surgical manipulations. Ureteral stents are typically about 30-cm long, hollow catheter-like devices composed of a polymer and placed within the ureter with the proximal end residing in the kidney and the distal end in the bladder. Ureteral stents function by channeling the flow of urine from the kidney to the bladder. One or both ends of a ureteral stent may be coiled in a pigtail shape to prevent the upward and/or downward migration of the stent due to patient movement. The ureter may stretch up to 5 cm in either direction during a patient's normal bodily movements, such as movement during breathing. If the stent is not sufficiently anchored, this may result in stent migration and displacement. Also, a stent may cause tissue irritation due to the relative movement between the stent and the ureter during natural stretching of the ureter, even when the stent is properly anchored. A typical semi-rigid, anchored stent is unable to adjust for the natural extension and contraction of the ureter during bodily movements, resulting in pressure and irritation of the ureter and surrounding tissue.
Regions of tissue most vulnerable to stent-induced irritation include the kidney, the renal pelvis, the sensitive bladder tissue in the trigonal region, and the tissue of the ureteral vesical junction leading into the bladder. Irritation may be caused by the static or dynamic contact of the semi-rigid stent with sensitive tissues of the body, such as the kidney and the renal pelvis. Chronic trigonal tissue irritation may result from contact of tissue by the bladder-anchoring features of the stent, for example, pigtails at the stent ends. Irritation problems are of concern regardless of the duration of use of the stent; however, irritation is of particular concern where use of a stent is required over a long period of time.
Another problem associated with ureteral stents is urine reflux and pain during urine voiding. On the initiation of voiding, the bladder wall muscles contract causing the pressure inside the bladder to increase. Because a typical ureteral stent holds the ureteral orifice open, increased bladder pressure during voiding is transmitted to the kidney through the stent, causing urine reflux and flank pain.